Compounds of the formulae 1, 2, and 3 in which R.sup.2 is hydrogen have been described by a number of authors, and a comprehensive review of the early work in this field may be found in "Cephalosporins and Penicillins", E. H. Flynn, Ed., Academic Press, New York and London 1972, p. 199 ff. Those latter compounds were initially obtained by treating a penicillin sulfoxide with a trialkylphosphite to obtain the corresponding compounds of formula 1 which, upon treatment with triethylamine, yielded the corresponding compounds of formulae 2 or 3. Micetich et al., Tetrahedron Letters 1976, 975 found that heating of certain 6-thioamides of penicillin sulfoxides gave the corresponding dithiazineazetidinones which yielded the corresponding compounds of formula 1 in which R.sup.2 is hydrogen upon exposure to light, moisture, and air. Similar compounds of formula 1 have also been obtained by Kukolja et al., U.S. Pat. No. 3,880,872, issued Apr. 29, 1975, by treating certain 4-chloroazetidinone-3-imidoylchlorides with metal sulfides. Franceschi et al., J. Am. Chem. Soc. 99,248(1977), treated certain compounds of formula 1 in which R.sup.2 is hydrogen with N-bromosuccinimide to obtain the corresponding compounds of formulae 2 and 3 in which R.sup.2 is bromine, converted them to the corresponding compounds in which R.sup.2 is acetoxy or methoxy which were obtained as mixtures of E+Z isomers, separated the latter compounds by chromatography, and cyclized them to mixtures of the correspondingly substituted .DELTA..sup.2 - and .DELTA..sup.3 -cephems by multistage procedures. The above processes are described in detail in Belgian Pat. Nos. 829,304, published Nov. 21, 1975; 830,089, published Dec. 11, 1975; and 832,424, published Feb. 16, 1976.